We intend to study the structure of membrane-bound cytochrome oxidase by low dose electron microscopy (LDEM) at 10 A resolution. The procedures make use of new specimen-preserving preparation techniques, induced peroidic arrangement of the enzyme in the membrane, subminimal exposure and computer techniques for reconstruction of projections and 3-d reconstruction of the undamaged protein structure. The location of the active sites and conformational changes will be investigated by difference maps, and the structure will be interpreted in terms of the biological functions of the enzyme. The proposed project develops the methodology of high resolution structural investigation of membrane-associated proteins by electron microscopy which will be of general use in the study of membrane functions.